Method for creating underground cavities employing explosives

ABSTRACT

Underground cavities are created by explosive springing wherein an explosive load is maintained below its critical amount by displacing a portion of the volume of a treatment cavity with a non-explosive medium.

M W J n w m D 1 w 9 O l 7 9 3 A ii Eitaies 2 mm 19 4 n M 9m 1.18 mm 41 w mL WP MP. W H WW n S MT m T. A P N m E .R O F France .{75} Inventor: Charles H. Grant, Midl d, Mi h 3 3/1955 [73] Assignec: The

Primary Examiner-Vcrlin R. Pendergrass Dow Chemical Company,

Midland, Mich.

Feb. 4,1971'- Al t0mey-Griswold & Burdick, Bruce M. Kanuch and Lloyd S .lowanovitz 221 Filed:

21 A LN 112704 I pp o ABSTRACT v [52] US. Undargmund cavities are Created byexploslve Spring i ingwvherein an explosive'lo'ad is maintainedbelow its Field of Search critical amount by displacing a portion of the volume of a treatment cavity with a non-explosive medium.

References Cited UNITED STATES PATENTS 6 Claims, 4 Drawing Figuiesv 3,064,572 W196i Aitchinson.............................102/23 PATENTEBM 16 I975 INVTOR. Char/6x5 H Granf I BY 644w MA BACKGROUND OF THE INVENTION Subterranean cavities are employed for many purposes. For example, they' are used to store chemicals and waste materials; they are employed to provide an area in which the surrounding formation may be treated, e.g. by displacing a soil stabilizing chemical into the formation or-the like; and they are also employ'ed in cratering programs wherein a relatively large amount of explosive is loaded into the cavity and detonated to create a crater of a predetermined size. Cratering methods are employed to create channels, for laying pipelines, to deepen harbors and for other like purposes.

Explosive springing is one method employed for creating such cavities. Generally, a borehole is drilled to a predetermined depth (about the depth of the cavity desired) and an explosive is loaded into the borehole and detonated to increase the size of the borehole at the location of the detonation to form a cavity. In the past, thistechnique has been limited to the preparation of relatively small cavities since as the cavity increases in size the amount of explosive which is loaded therein approaches what is known in the art as the critical amount (weight or volume) of that explosive at the depth at which it is to be detonated. Upon the detonation of a greater than critical amount of an explosive the surface of the earth will be fractured. Thisis un-' desirable where a cavity is desired instead of a crater.

The present invention concerns a method for creating underground cavities employing a sequence of explosive springing steps.

The critical depth of an explosive as employed herein is a parameter which indicates how far down in a borehole (i.e., depth below the surface) a given amount (volume or weight) of a certain explosive can be detonated without breaking the surface of the earth. 40

The critical depth of a given amount of any particular explosive in any particular type of formation may be experimentally determined by a series ofdetonations of a certain amount of that explosive in the particular formation; each detonation being made at a different depth. The depth (location of the center of gravity of the explosive load) at which the explosive upon detonation no longer breaks the surface of the earth is considcred the critical depth of that amount of explosive in that type of formation.

Two procedures have been developed in the art for experimentally determining the critical depth of an explosive. These procedures are explained in detail in the article by Charles H. Grant Simplified Explanation of Crater Method," Engineering and Mining Journal, Vol. 65, No. ll, pages 8689, Nov., 1964. As more fully explained in the Grant article, C. W. Livingston determined that there was a constant factor between the critical depth and the cubed root of the weight of the explosive. He expressed this relationship by the formula N EW In this formula E is called the strain energy factor (or a weight crater constant), N is the critical depth (to center of gravity of explosive load) and W is the weight of the explosive. By experimentally determining the critical depth N for any particular explosive you then solve the above formula to determine the constant E for that particular explosive. Having determined the strain energy factor E for that particular explosive in that type of formation you can then solve for the critical depth of any other given weight of that particular explosive for use in that type of formation. Or

you can determine the critical weight (W) of explosive for any given depth (N).

The other procedure developed makes a comparison of explosives on. the basis of a constant explosive volume and shape, rather than on a constant weight ba sis. As in the weight crater method the critical depth is determined as the depth of an explosive load where the. I

center of gravity of explosive load), I is the volume crater constant and Vis the volume of the explosive. A

further refinement of this formula is also set forth in the above article. An additional parameter A was developed to express the ratio of the depth (d) of the center of gravity ofa certain volume of an explosive in the borehole to the experimentallydetermined critical depth (N) of that explosive (A=d/N). When the depth (d) of a particular explosive is exactly equal to the experimentally determined critical depth (N) a crater 'will not be formed (A is equal to 1). If thecenter of gravity 0 of a particular explosive is at a depth less than the value of (N), A is less than 1, a crater will be formed. When (d) is below the critical depth (N), A is greater than I no'crater Will'be fomred'." "This-"dimensionless parameter A isemployed as an aid to determine at what depth a certain amount of an explosive can be loaded without breaking the surface of the ground. y

7 Therefore, when E and N have been determined for a certain explosive, the depth (d) at which a certain volume of the explosivemust be loaded so as to not break the surface of the earth is calculated by solving 7 the formulas A== d/N and d=A N=A 2V wherein A is equal to or greater than I and E and V are known for the explosive.

7 Likewise, one may calculate what volume of exploscope of the invention to the theories expressed herein.

However, these formulas and calculations are useful for practicing the principles of the present invention and theories and formulas may be developed to determine the critical depth or amount of any particular explosive in a particular type of formation and could likewise be employed. The above discussion further emphasizes the fact that the present invention is not limited to use of any one particular explosive composition for practicing the invention. The critical depth of any particular explosive composition is merely determined in the type of rock in which the cavity is to be formed. Having determined this, the volume or weight of explosive which will break the surface of the earth when detonated at any particular depth can be readily determined. When the size of the cavity approaches the size which will hold an amount (volume or weight) of explosive which will, upon detonation, break the surface of the earth, the principles of the present invention'are employed to allow the further expansion of the cavity.

SUlvil-vlARr' OF THE INVENTION In the practice of the present invention wherein an explosive springing method is employed to create an underground cavity, when the size of the cavity is great enough to hold an amount (weight or volume) of an explosive which upon detonation will break the surface of l the earth, i.e. the critical depth of that amount of explosive has been reached, a sufficient amount of the void of the cavity is filled with a nonexplosive medium to decrease the available volume of the cavity to an amount below the critical amount of the explosive. The remaining volume contains the explosive which is detonated to further increase the size of the cavity without breaking the surface of the earth. The

procedure maybe repeated any number of times to create a cavity of a desired size.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 14 are diagramical views showing the creation of a cavity having a size about equal to that shown by the dotted line employing the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In practicing the present invention, the amount of explosive, i.e. weight or volume, which will cause the sur.

" f aceoh the sorth to-fail whenvd-ctonated iatta-predexen mined depth (Le. the depth of the center of gravity of the desired cavity) is first determined. The critical depth of the explosive composition to be employed can be determined experimentally be detonating a series of explosive loads at different depths or it may be readily available from data generated in ihe art or from the manufacturer of the explosive being'employed.

A borehole is drilled to the approximate depth of the desired cavity. The lower portion of the borehole is then filled with the explosive to a height about equal to the desired height of the cavity anddetonatcd to create an initial treatment cavity. In this embodiment is is assumed that the initial borehole is not large enough to contain a greater than critical amount of explosive. If, however, it is so large the principles of the invention are employed when the borehole is loaded with the first explosive charge. Rubble from the detonation is then usually removed from this cavity and the cavity is again filled with the explosive and again detonated. This procedure is continued until the treatment cavity approaches a size which will hold an amount of explosive which is sufficient upon detonation to break the surface of the earth. A portion of the void of the treatment cavity is then occupied by a non-explosive medium,

preferably in a manner such that the explosive which is loaded therein is held against at least a portion of the inner wall of the cavity. The medium is employed in an cavity is created or the amount of explosive remainin after incorporating the medium is no longer sufficient to increase the size of the cavity.

The principles of the present invention are illustrated in FIGS. 1-4. It is desired to create a cavity of the size about equal to that shown by the dashed line 10. A

borehole ll of any convenient size is drilled from the surface of the earth to about the lower'portion of the desired depth of the cavity. An explosive 12, having known characteristics, i.e. the critical depth thereof and theamount which will break the surface of the earth at the depth of the cavity has been determined, is

placed in the borehole to a height slightly less than the height of the desired cavity. The explosive 12 is armed with a suitable initiator 13 which in turn is connected to; 'for'example, ahigh velocity detonatin g cord '1 land the detonating cord in turn connected to a detonator, not shown. The explosive load 12 is then usually stemmed with a suitable stemming material 15, e.g.,

' drill cutting, sand, pea gravel or the like. The explosive load is detonated creating an initial cavity 16 (FIG. 2). Any rubble caused by the detonation of the explosive is usually removed from the cavity 16. If size of the cavity 16 is sufficiently small, i.e. if when completely filled with the explosive and detonated, the surface of the earth will not be fractured, it is agaittfilled with an explosive which is armed and detonated to further enlarge the cavity. When the size of the cavity, however, approaches that which, when entirely loaded with the V explosive and detonated, would fracture the surface of the earth a portion of the next explosive load is displacethby a nonexplosi-vcmediurn.

In the embodiment shown in FIG. 3, a portion of the volume of the cavity is occupied by an inflatable device 17, for example, a bag made from rubber of the like.

after the explosive is loaded into the borehole. The

amount to sufficiently decrease the size of the available volume of the cavity so that upon the detonation of the explosive loaded into the available space the surface of the earth will not be broken. The explosive charge is then detonated to further increase the size of the cavity. This procedure is continued until the desired size exact sequence is not critical to the practice of the present invention. The explosive is then again armed and the borehole stemmed. Upon detonation of the explosive the cavity is further increased in size without breaking the surface of the earth. This procedure is repeated' any number of times until a cavity of a desired size is prepared.

The previous embodiment illustrates the useof an inflatable device 17 to displace a portion of the explosive loaded into the cavity. Other means may also be employed to produce this desired effect. For example, an explosive may be placed in an expandable package having an initial dimension such that it can be lowe 'fi down the borehole and into the cavity. The packet. should also include therein a material, such as for example, an expandable fluid, e.g. contained in a second inflating bag (e.g. CO or the like), which is activated or'is self-activating to expand and force the explosiv against at least a portion of the inner wall of the cavity- In another embodiment a portion of the volume 0 the of formation in which the cavity is to be created can be readily determined by those skilled in the explosive art.

'For example, metallized explosives well known in the art, e.g. metallized ANFO, and slurry explosives containing an inorganic oxidizing salt,'water, a particulate metal and a thickening agent, can be employed. Exampics of specific explosives which can be employed are taught in U.S. Pat. Nos. 3,094,069; 3,307,986;

Moreover, the same explosive composition need not be employedineae'h :shot toincrease the size-ofthe cavity. For example, a very powerful explosive (e.g.. highly metallized) may be employed in the first shot (or first few shots) and. when the size of the .cavity ap proaches that, where if the powerful explosive were about 40 inches in diameter and 10 feet high is made in a subterranean formation at a depth of about feet (to floor of chamber). A 6 inch diameter borehole issunk to a depth of '35 feet in the area where the chamber is desired. An explosive is employedwhich has a sigma value 2 see previous discussion) of about l in the formation in which the chamber is to be formed. TheexpIosive -When detonated -in-the--forma- The volume of explosive which the 9 foot by 6 inch diameter borehole holds is-about 3053 cu./in. The critical depth (N= Ev for this amount ofexplosive is 14.5

ft. or well above the 30.5 ft. center of gravity of the explosive load A d/N 2.1) so the ground will not break at the surface. The hole is loaded to a height of 9 feet with the explosive and detonated. A chamber about 24 inches in diameter and 10 feet high is produced.

The volume of the so-produced chamber is equal to about 54,400 cubic inches (or 31.6 cu. ft.). The critical depth of this volume (54,400) of explosive (N 2 v is about 38 ft. or well below the center of gravity'of the explosive load of 30 fee't( A 0.79) and the surface of the earth would be fractured if this volume of explosive v i 30 feet is 8000 cubic inches. This leaves 46,400 cubic inches of cavity space which must be filled. This space is filled by inflating a resilient rubber container in the chamber with water to compress 8000 cubic inches of the explosive against the inner wall of the chamber.

The explosive is detonated and the size of the chamber is increased without fracturing the surface of the earth.

This procedure is repeated, always keeping the critical volume of explosive below that which will fracture the surface of the ground, until the chamber 40 inches which is smaller than the desired cavity, said treatment cavity being located at a depth about equal to, the

'desir'ed undergrouhd cavity; and the explosive is detonated to increase the size of the treatmentcavity, the improvement which comprises:

a. determining the critical amount of said explosive whichv/hemdetonated atsaiddepth willcause the surface of the earth to fail;

b. enlarging said treatment cavity until said cavity is sufficiently large to hold said critical amount of said explosive;

c. displacing a portion of the volume of the enlarged treatment cavity with an amount of a nonexplosive medium sufficient to decrease the volume of the cavity to an amount which will not contain said critical amount of the explosive;

d. loading said remaining volume of said treatment cavity with an explosive; and

e. detonating said explosive to further increase the size of said treatment cavity without breaking the surface of the earth.

2. The improved method as defined in claim 1 wherein the explosive is loadedintothe cavity, a,por-

tion 'of the volume of which is occupied by said nonexplosive medium, and against at least a portionof the inner wall thereof.

4 3. The improved method as defined in claim 2 wherein the nonexplosive medium comprises an inflatable device.

4. The improved method as defined in claim 1 I wherein a portion of the cavity is displaced by a nonexpanding fluid into said expandable package, and forcplosive medium by loading into said treatment cavity at least one expandable package having incorporated therein said explosive and an expanding fluid in an amount which is sufficient to decrease the volume of the treatment cavity below the critical volume of the explosive when said fluid is expanded, releasing said ex ing said explosive against at least aportion of the inner wall of the cavity. 5 y

5. The improved method as defined in claim 1 including the steps of:

a. lowering an inflatable container ment cavity, I

b. loading a volume of said explosive into said treatinto said treatment cavity said volume being below critical volume of said explosive, and

' c. inflating said container to a size sufficient to force said explosive against at least a portion of the inner wall of said cavity. I

6. The improved method as defined in claim 1 including the steps of:

a. lowering an inflatable container into said treatment cavity, b. inflating said container to a size sufficient to decrease the volume of the treatment cavity below the critical volume of the explosive, and

said treatment 

1. In the method of creating an underground cavity of such a size that the surface of the earth will fail if the entire cavity is filled with an explosive and detonated wherein an explosive is loaded into a treatment cavity which is smaller than the desired cavity, said treatment cavity being located at a depth about equal to the desired underground cavity, and the explosive is detonated to increase the size of the treatment cavity, the improvement which comprises: a. determining the critical amount of said explosive which when detonated at said depth will cause the surface of the earth to fail; b. enlarging said treatment cavity until said cavity is sufficiently large to hold said critical amount of said explosive; c. displacing a portion of the volume of the enlarged treatment cavity with an amount of a nonexplosive medium sufficient to decrease the volume of the cavity to an amount which will not contain said critical amount of the explosive; d. loading said remaining volume of said treatment cavity with an explosive; and e. detonating said explosive to further increase the size of said treatment cavity without breaking the surface of the earth.
 2. The improved method as defined in claim 1 wherein the explosive is loaded into the cavity, a portion of the volume of which is occupied by said nonexplosive medium, and against at least a portion of the inner wall thereof.
 3. The improved method as defined in claim 2 wherein the nonexplosive medium comprises an inflatable device.
 4. The imprOved method as defined in claim 1 wherein a portion of the cavity is displaced by a nonexplosive medium by loading into said treatment cavity at least one expandable package having incorporated therein said explosive and an expanding fluid in an amount which is sufficient to decrease the volume of the treatment cavity below the critical volume of the explosive when said fluid is expanded, releasing said expanding fluid into said expandable package, and forcing said explosive against at least a portion of the inner wall of the cavity.
 5. The improved method as defined in claim 1 including the steps of: a. lowering an inflatable container into said treatment cavity, b. loading a volume of said explosive into said treatment cavity said volume being below critical volume of said explosive, and c. inflating said container to a size sufficient to force said explosive against at least a portion of the inner wall of said cavity.
 6. The improved method as defined in claim 1 including the steps of: a. lowering an inflatable container into said treatment cavity, b. inflating said container to a size sufficient to decrease the volume of the treatment cavity below the critical volume of the explosive, and c. loading the remaining volume of said treatment cavity with said explosive. 